Polymer-coated Fiber Bragg Gratings Research Articles

Simultaneous detection of carbon dioxide and relative humidity using polymer-coated fiber Bragg gratings

Carbon dioxide (CO2) sensor has a wide range of applications in many industrial fields. However, most existing CO2 sensors exhibit obvious cross-sensitivity to environmental relative humidity (RH). Therefore, it becomes critical for CO2 sensors to eliminate the interference of humidity. In this study, based on the advantages of conveniently constructing sensing network and multiplexing capability of FBGs, the combination of polyimide-coated fiber Bragg gratings (FBGs) and etched FBGs sensors with different sensitivities was proposed to detect the concentration of CO2 and humidity levels simultaneously. Two sets of sensors were fabricated by changing the coating thickness and etching the grating region, respectively. The experimental results indicate that all the sensors have a good linear response to CO2 and humidity, as well as excellent response reversibility and repeatability. The highest sensitivity of the sensors towards CO2 was 0.585 με/vol% CO2, while 4.191 με/% RH towards humidity. The CO2+RH mixture exposure experiments were carried out to demonstrate the feasibility of simultaneous detection of CO2 and humidity levels, which showed small deviations between the detected and set values of CO2 concentrations and humidity levels. Moreover, the fabricated sensors can operate over a wide range of temperatures.

Polymer-coated FBG humidity sensors for monitoring cultural heritage stone artworks

Relative Humidity (RH) sensors based on Fiber Bragg Grating (FBG) technology have been fabricated by polymer functionalization, using hygroscopic material, namely agar. The sensor is specifically intended to monitor moisture in stones of cultural heritage buildings and monuments, in particular to be positioned under hydrophobic coatings used for protecting stones from absorption of water and dissolved chemicals, thus providing early warning of coating deterioration by RH monitoring. The performance of the prototype was first tested exposing the sensors in controlled environment and then assessed for the application simulating “in field” operating conditions, with sensors installed on marble and travertine tiles. Results show that the proposed sensor is able to monitor the presence of moisture inside the stones.

Polymer-coated FBG sensor for simultaneous temperature and strain monitoring in composite materials under cryogenic conditions.

A polymer-coated fiber Bragg grating (PCFBG) is examined for real-time temperature and strain monitoring in composite materials at cryogenic temperatures. The proposed sensor enables the simultaneous measurement of temperature and strain at extremely low temperatures by tracking the changes in the reflected center wavelengths from a pair of PCFBGs embedded in a composite material. The cryogenic temperature sensing was realized by introducing polymer coatings onto bare FBGs, which resulted in high temperature sensitivity under cryogenic conditions. A comparison of wavelength responses of the Bragg grating with and without a polymer coating toward temperatures ranging from 25°C to -180°C was performed. The polymer-coated FBG exhibited a sensitivity of 48pm/°C, which is 10 times greater than that of the bare FBGs. In addition, the encapsulation of the FBG in a capillary tube made it possible to evaluate the strain accumulated within the composite during operation under cryogenic conditions.

Fiber Bragg Grating sensors to measure the coefficient of thermal expansion of polymers at cryogenic temperatures

One of the fundamental advantages in the employment of the Fiber Bragg Grating (FBG) sensors lies in their capability to allow measurements in extreme environmental conditions with also very high immunity toward external electromagnetic interference factors.The behavior of a polymer-coated FBG sensor, tested in cryogenic conditions at the laboratories of the European Organization for the Nuclear Research (CERN) in Geneva, has been analyzed and will be discussed in this paper. Magnets used in the Large Hadron Collider (LHC) for the High Energy Physics Researches at CERN need in fact extreme cooling conditions to preserve the internal superconductivity highly crucial for their performances. The magnets, built with NbTi based superconductors, are cooled with liquid helium and they operate at 1.9K. The aim of the present work is to estimate the thermal expansion coefficient of two polymers based on epoxy and methacrylate (PMMA) used as coating of FBGs, in the temperature range from 4K to 300K and to check their suitability for the use in temperature monitoring of the superconducting magnets. A standard numerical derivative method has been employed to estimate thermal expansion coefficients; moreover the correlated fluctuation analysis (CFA) based procedure is proposed, as a reliable alternative, to overcome numerical derivative drawbacks at very low temperatures within the range of 4–20K. The calculated values of thermal expansion coefficient for both systems are in agreement with literature data on similar material.

Polymer‐coated fiber Bragg Grating Sensor for cryogenic temperature measurements

AbstractCryogenic temperature sensing has been demonstrated using Polymer coated Fiber Bragg Grating Sensor, and the experimental results are presented in this article. Teflon coating thicknesses of 20 and 40 μ applied on Fiber Bragg Gratings, inscribed in silica fiber (TFBG) are used for these studies. It was observed that the sensitivity of Teflon‐coated FBG has been enhanced three times compared to uncoated regular FBGs. © 2011 Wiley Periodicals, Inc. Microwave Opt Technol Lett 53:1154–1157, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.25914

UV Sensor Based on Photomechanically Functional Polymer-Coated FBG

A simple ultraviolet (UV) sensor based on a photomechanical responsible material (azobenzene moiety) and a typical fiber Bragg grating (FBG) has been proposed and demonstrated for the first time. This sensing device, called the azobenzene-coated FBG sensor, can easily measure the UV light intensity by determination of its center wavelength shift which results from the photoisomerization of the azobenzene moieties. We show that the peak wavelength can be rapidly shifted up to 0.6 nm because of the photomechanical stretching effect of the azobenzene-coated FBG. The strain applied to the FBG under a UV intensity of 208 mW/cm2 was estimated to be 0.054%. We have also confirmed the linearity and stability of our proposed UV sensor.

Polymer coated fiber Bragg grating thermometry for microwave hyperthermia

Measuring tissue temperature distribution during electromagnetically induced hyperthermia (HT) is challenging. High resistance thermistors with nonmetallic leads have been used successfully in commercial HT systems for about three decades. The single 1 mm thick temperature sensing element is mechanically moved to measure tissue temperature distributions. By employing a single thermometry probe containing a fixed linear sensor array temperature, distributions during therapy can be measured with greater ease. While the first attempts to use fiber Bragg grating (FBG) technology to obtain multiple temperature points along a single fiber have been reported, improvement in the detection system's stability were needed for clinical applications. The FBG temperature sensing system described here has a very high temporal stability detection system and an order of magnitude faster readout than commercial systems. It is shown to be suitable for multiple point fiber thermometry during microwave hyperthermia when compared to conventional mechanically scanning probe HT thermometry. A polymer coated fiber Bragg grating (PFBG) technology is described that provides a number of FBG thermometry locations along the length of a single optical fiber. The PFBG probe developed is tested under simulated microwave hyperthermia treatment to a tissue equivalent phantom. Two temperature probes, the multiple PFBG sensor and the Bowman probe, placed symmetrically with respect to a microwave antenna in a tissue phantom are subjected to microwave hyperthermia. Measurements are made at start of HT and 85 min later, when a 6 degrees C increase in temperature is registered by both probes, as is typical in clinical HT therapy. The optical fiber multipoint thermometry probe performs highly stable, real-time thermometry updating each multipoint thermometry scan over a 5 cm length every 2 s. Bowman probe measurements are acquired simultaneously for comparison. In addition, the PFBG sensor's detection system drift over 10 h is measured separately to evaluate system stability for clinical applications. The temperature profiles measured by the two probes simultaneously under microwave HT are in good agreement showing mean differences of 0.25 degrees C. The stability of the detection system is better than 0.3 degrees C with response times of the PFBG sensor system of 2 s for each scan over ten points. The single fixed multipoint fiber thermometry capability compares favorably with the scanning Bowman probe data. This offers an enabling alternative to either scanning or bundled single point temperature probes for distributed thermometry in clinical applications.

Polymer-Coated Fiber Bragg Grating Sensors for Simultaneous Monitoring of Soluble Analytes and Temperature

A new fiber-optic sensor for simultaneous measurement of water-soluble analytes and temperature with polymer-coated fiber Bragg gratings (FBGs) is proposed. As an application of the approach, simultaneous monitoring of the concentration of sugar or potassium chloride (KCl) and temperature has been achieved. Changes in these environmental parameters result in different extents of either red- or blue-shifts of the Bragg resonance wavelengths of the gratings. It has been found that polyimide-coated FBG responds to variations of both temperature and concentrations of soluble analytes, while acrylate-coated FBG is sensitive to environmental temperature only. The experimental results showed that the temperature sensitivity of the acrylate-coated FBG, temperature, sugar, and KCl concentration sensitivities of the polyimide-coated FBG are 0.0102 nm/degC , 0.0094 nm/degC, 0.0012 nm/degBx , and 0.0126 nm/M, respectively. The sensing mechanism of the polyimide-coated FBG lies in the hygroscopic properties of the polyimide coating, which result in the change of the strain of the fiber and, thus, the optical properties of the grating. Since the sensor detects the analytes that swell the polyimide coating and different analytes induce different swelling effects, the sensor can detect different analytes without prior knowledge once a calibration curve is developed.

Tuning the sensing responses of polymer-coated fiber Bragg gratings

We demonstrate the application of polyimide-coated fiber Bragg grating (FBG) as a substance sensor and reveal the dependences of the sensitivity and time response on the coating thickness as well as factors influencing these properties. As an example of polyimide-coated sensor for detecting soluble substances, the use of the grating for salinity measurement shows that a judicious selection of coating thickness is needed in order to achieve suitable sensitivity and time response for specific application. The experimental results indicated that the salinity sensitivities of the polyimide-coated FBGs with coating thicknesses of 11, 17, 20, and 24 μm were 1.45, 2.45, 2.95, and 3.70 pm/% (blueshifted), respectively. In addition, the response time is another signature for discriminating different parameters.

A multiplexed fiber Bragg grating sensor for simultaneous salinity and temperature measurement

An in-line one-fiber approach to realize simultaneous measurement of salinity and temperature is proposed. The sensor system, which consists of multiplexed polymer-coated fiber Bragg gratings, showed that the polyimide-coated grating responds to variations of both temperature and salinity, while the acrylate-coated grating is only sensitive to the environmental temperature. The experimental results indicated that the temperature sensitivity of the acrylate-coated grating in water was 0.0102nm∕°C for redshifted Bragg wavelength with increasing temperature, and the temperature and the salinity sensitivities of the polyimide-coated grating were 0.0094nm∕°C (redshifted) and 0.0165nm∕M (blueshifted), respectively, which are in excellent agreement with the theoretical analysis.